The ability of arterial spin labeling (ASL) to detect perfusion abnormalities in clinical populations can be limited by poor signal to noise and transit-time artefacts. This is evident in studies involving frontotemporal dementia patients, where reports on the diagnostic value of ASL have been inconsistent. This study presents a head-to-head comparison of regional hypoperfusion detected by ASL and PET with radiolabeled water (15O-water), the gold standard for measuring CBF. T-maps depicting hypoperfusion were generated using absolute and relative CBF. There was good agreement between regional hypoperfusion identified by ASL and 15O-water, particularly for relative CBF maps which reduced inter-subject variability.